consistent protcol definition

Author: maxfischer2781

asyncstdlib/_typing.py

@@ -58,7 +58,8 @@
 
 # bool(...)
 class SupportsBool(Protocol):
-    def __bool__(self) -> bool: ...
+    def __bool__(self) -> bool:
+        raise NotImplementedError
 
 
 # LT < LT